The present invention relates generally to power amplification circuits, and more particularly, to an improvement in a circuit for performing a switching operation in response to pulse width modulation (hereinafter, referred to as PWM) signals based respectively on a plurality of audio signals or the like so as to cause the PWM signals to be subjected to power amplification, and then supplying circuit loads each including a speaker or the like with a plurality of output signals obtained with the power amplification.
In the field of acoustic apparatus operative to amplify an audio signal and supply a speaker with the amplified audio signal to obtain reproduced sound based on the audio signal, there have been proposed various systems for amplifying the audio signal in pursuit of the respective purposes assigned thereto. In particular, with respect to power amplification performed to obtain, based on an input audio signal, an output audio signal used for driving a speaker, a D-class amplification which is carried out by an active amplifying element, such as a transistor, functioning in the manner of D-class operation, is often adopted because a relatively superior distortion characteristic can be obtained thereby.
D-class amplification is generally performed with a switching operation of an active amplifying element, for example, a transistor, in response to an input signal which is an audio signal or the like. In the case of a power amplification circuit which performs D-class amplification for audio signals, there previously has been proposed such a circuit that is operable first to produce a PWM signal based on an input audio signal, then to cause the PWM signal to be subjected to power amplification and to supply a speaker portion with the amplified PWM signal through a low pass filter (hereinafter, referred to as LPF), as disclosed in, for example, Japanese patent application published before examination under publication number 2002-158544.
The power amplification circuit previously proposed is configured to be a so-called Balanced Transformerless (hereinafter, referred to as BTL) type, namely, a power amplification circuit in which a couple of switching amplifier portions are provided for driving a speaker portion connected in common to the switching amplifier portions. In such a power amplification circuit, a pulse width modulation amplifier is provided for producing first and second PWM signals having complementary variations in the respective pulse widths caused in response to a digital signal in an audio frequency band as the input audio signal. The first PWM signal obtained from the pulse width modulation amplifier is subjected to power amplification carried out with the switching operation of a first power switching circuit (a first switching amplifier portion) performed in response to the first PWM signal, and a first PWM power signal which is obtained as an output signal of the first power switching circuit is supplied to a first power LPF. The second PWM signal obtained from the pulse width modulation amplifier is subjected to power amplification carried out with the switching operation of a second power switching circuit (a second switching amplifier portion) performed in response to the second PWM signal, and a second PWM power signal which is obtained as an output signal of the second power switching circuit is supplied to a second power LPF.
Then, a couple of signals derived, respectively, from the first and second power LPFs, which are opposite one another in polarity, are supplied to the speaker portion connected to both of the first and second power LPFs. As a result, the speaker portion is driven differentially with the signals opposite one another in polarity to reproduce sound in response to the input audio signal.
In the previously proposed power amplification circuit mentioned above, the first and second PWM signals obtained from the pulse width modulation amplifier based on the digital signal in the audio frequency band as the input audio signal are usually produced with the use of a common clock signal. Each of the first and second PWM signals produced with the use of the common clock signal has substantially the same carrier frequency corresponding to the frequency of the clock signal and therefore the first and second PWM signals have respective periods synchronized with one another. Then, a first switching operation is performed at every period of the first PWM signal in the first power switching circuit responsive to the first PWM signal and a second switching operation is performed at every period of the second PWM signal in the second power switching circuit responsive to the second PWM signal.
In each of the first and second power switching circuits, an overshooting or undershooting variation caused by ringing or the like appears on the rising or falling edge of the first or second PWM power signal whenever the switching operation is performed in response to the first or second PWM signal. The overshooting or undershooting variation thus appearing on the rising or falling edge of the first or second PWM power signal results in undesirable noise.
Since the switching operation of each of the first and second power switching circuits is performed at every period of one of the first and second PWM signals, the noise resulting from the switching operation of the first power switching circuit appears at every period which is the same as the period of the first PWM signal and the noise resulting from the switching operation of the second power switching circuit appears at every period which is the same as the period of the second PWM signal. Accordingly, the noise resulting from the switching operation of the first power switching circuit and the noise resulting from the switching operation of the second power switching circuit each contain a main frequency component, the frequency of which is the same as the carrier frequency of each of the first and second PWM signals.
The first and second power switching circuits constitute a switching amplifier operable to cause each of the first and second PWM signals to be subjected to power amplification. Although the switching amplifier thus configured is provided to be a single amplifier for an audio signal of a single channel in the case of a single-channel acoustic apparatus, a plurality of switching amplifiers are provided in parallel for a couple of stereophonic sound signals or three or more multi-channel sound signals in a multi-channel acoustic apparatus. In the case where a plurality of switching amplifiers are provided in parallel, each of the switching amplifiers generates noise containing a main frequency component having a frequency which is the same as the carrier frequency of each of the first and second PWM signals.
Under such circumstances, a plurality of pairs of the first and second PWM signals supplied to the plural switching amplifiers as input signals are usually produced with a common clock signal so as to have the same carrier frequency. Therefore, the noise generated by each of the switching amplifiers has a main frequency component which is substantially the same as the carrier frequency of each the first and second PWM signals. This means that the noise generated by each of the switching amplifiers is concentrated upon a particular relatively narrow frequency band and it is feared that the noise will grow to be so large as to be transmitted to the outside as unwanted radiation to disturb communications on the outside and further to exert a bad influence upon electronic apparatus used on the outside.